According to Lyell, there was around the base of the cone, spreading from them as a centre over an area of some four square miles, a convex mass, about 550 feet in height, most of the surface of which was covered with thousands of small flattish conical mounds from six to nine feet in height. These, together with numerous large fissures that crossed the plain in different directions, served as points for the escape of sulphur vapors, as well as for the vapors of hot water.
During the escape of lava from the craters in 1759, the molten rock, spreading over the plain, ran into the channels of the river or brooks before named, driving out the water. This water reappeared at the base of the mountain in numerous hot springs.
Humboldt thought that the conical mountains had been lifted or raised by the formation of huge bubbles formed under the lava, thus causing it to assume a shape not unlike that of a huge bladder. This opinion, however, has not been accepted by geologists at the present time. Scrope points out that this was probably the origin of the little conical mounds that covered the surface of the principal conical mounds but was not, in all probability, the cause of the mound itself. He says:
"With regard to the disputed question as to the origin of the raised plain of the Malpais, M. de Saussure, the last and most trustworthy visitor, entirely confirms the opinion which I ventured to proclaim in 1825, that Humboldt was mistaken in supposing it to have been 'blown up from beneath like a bladder,' and that it is merely an ordinary current of lava, which, owing to its very imperfect liquidity at the time of its issue from the volcanic vent, as well as to the overflow of one sheet or stream upon another, had acquired great thickness about its source, gradually thinning off towards the outer limit of the elliptical area it covered."
If you have been able to follow the above you will see that Mr. Scrope means that in his opinion the cone of Jorullo is a lava cone like that we have already studied on Mt. Loa or Mt. Kilauea, or, in other words, that the lava as it came out from the opening on the top of Jorullo, flowed in all directions around the opening, thus building up a mountain in the form of a flat lava cone.