Extinct Volcanoes in Colorado. — Lakes. 39 
have reached the surface. These have sent off from the main 
dykes, intrusive sheets between lines of weakness in the strat- 
ification. Of this type are the pophyries of the Leadville and 
South Park region. A third class closely related to the last, 
have not only sent out intrusive sheets, but have arched up the 
overlying strata into oven-shaped cavities, which they have filled 
with massive lava, forming "laccolites" sometimes several 
thousand feet in thickness. When afterwards the arched 
strata overlying these laccolites have been removed by erosion, 
these great lava reservoirs have been exposed, and now form 
noted mountain peaks such as the Spanish Peaks near Trini- 
dad and Sopris, Gothic and Crested Butte mountains of the 
Elk range. In all these cases no evidence remains of the 
former existence of true volcanic craters. 
Amongst the summits of our mountains, we often notice 
deep punch-bowls or "cirques" simulating the form of volcanic 
craters. These are the work of erosion by water or glaciers, 
and are not of igneous origin. 
In New Mexico, amongst the basaltic table-lands, som.e true 
volcanic craters occur. 
Near Albuquerque in New Mexico and along the line of the 
Santa Fe railroad in Arizona, the traveler may observe flows 
of black, slaggy lava, in ropy coils, of exceedingly recent ap- 
pearance, with little or no vegetation growing over them. 
These flows occupy the modern valleys and dry river courses. 
They are evidently of comparatively recent origin, and might 
possibly be traced to extinct craters further back in the moun- 
tains. In the Costilla range on San Luis park some small 
craters are said to exist. About four miles to the west of the 
little town of Antonito, on the line of the Rio Grande railway 
in San Luis park, just on the edge of the Conejos mountains 
(a range composed of enormous masses of volcanic breccia 
and lava flows resting upon granite) a small conical shaped 
mountain may be seen resting on a plateau. The Conejos 
river cuts through this plateau, and shows its structure to 
consist of successive flows of black, basaltic lava. The pecu- 
liar shape of the mountain, a low cone, with a very broad base 
(the angle of convergence towards the cone being about five or 
more degrees) suggests a central vent, from which lava flows 
have poured down for some miles over the surrounding flat 
country. The banks of the railway track are covered with 
