138 THE AMERICAN NATURALIST. [VoL. XXXII. 
found augite-diorite, porphyrites, lamprophyres, and basalts. These 
are arranged as follows: plutonic rocks in the central core, porphy- 
ritic rocks in intruded sheets and dikes, extrusive rocks in lava 
flows, and tuffs and breccias underlying the lavas. All these differ- 
ent rocks are described in detail by Weed and Pirsson.' The 
plutonic rocks are granite and augite-diorite. They have altered 
the shales through which they intrude into tough hornstones, and 
the limestones into coarsely crystalline marbles containing in places 
garnet, phlogopite, vesuvianite and pyroxene. 
The naming of the rock types discussed by the authors is based 
on their macroscopic texture. In the acid series, for instance, those 
rocks are called granite which appear holo-crystalline to the naked 
eye. Those are named quartz-porphyry which possess a groundmass 
so finely granular that its components cannot be distinguished with- ' 
out the aid of a microscope, and those that contain glass or a very 
dense groundmass are called rhyolite. In their description of the 
granite-porphyry the authors describe the micro-pegmatitic structure 
as an original one and express doubts as to its ever being second- 
ary. Dark basic concretions in the granite are regarded as the 
result of the liquation of the liquid magma from which the rock 
solidified. One of the most interesting of the quartz-porphyries 
described is a rock containing sufficient tourmaline to rank as an 
accessory component. This mineral occurs in stellate groups re- 
placing feldspar. Very frequently fluorite is associated with it. An 
analysis of this rock gave : 
SiO, TiO, AlO, FeO; FeO MnO MgO CaO K,O NaO Li,0 KH,O Total 
82 13.80 37 10. = 
The lamprophyres cutting the “belt series” of shales, etc., are 
augite, — vogesites, minettes, monchiquites, and diabases. Many of 
the rhyolites are devitrified and many of them contain spherulites. 
These are thought in all cases to be original, 
A comparison of the analyses of the different rock types of this 
district shows that no absolute relation exists between the silica 
and the different bases; but it seems to indicate that there is a 
definite relation between the quantities of soda and potash present. 
The differentiation of the Castle Mountain series appears to have 
been deep-seated. 
The igneous rocks of the Denver Basin, Colorado, are principally 
basalts, which appear on the plains at the foot of the mountains as 
dykes and surface flows or sheets. Augite-syenite and quartz- 
1 Bulletin 139 of the U. S. Geol. Survey. Washington, 1896. 
