DIABASE OF TASMANIA. ZO tt 
Colorado, we read—‘ These great masses (of porphyrite) le 
in the comparatively soft Cretaceous strata, and erosion has 
removed the everlying sediments entirely or in great part. 
From the average elevation of the contact to the mountain 
summits is usually from 2000 to 3000 feet. The thickness 
now seen in no case represents the original vertical dimen- 
sions of the mass.” The Ragged Mountain is described as 
being about eight miles long. ‘On the west is a very 
abrupt wall more than 2000 feet in height, presenting an 
unbroken front for nearly three miles.’ The Anthracite 
Range is “a porphyrite mass four miles long from east to 
west, and quite narrow, possessing a rather sharp crest with 
several peaks, the highest of which has an elevation of 12,251 
feet. To the south this range presents an abrupt face for 
its entire length. The average height of this steep front 
is from 1500 to 2000 feet.”” Mount Carbon is “a single 
mass of eruptive rock, which rises from 2000 to 3000 feet 
above the contact with the surrounding Laramie strata.” 
Of Mount Wheatstone the report says—‘‘ This porphyrite 
mass is 44 miles in length, its breadth from two to three 
miles, and the summits are 3000 feet above the eastern sedi- 
mentary contact.’”’ Of Crested Butte, “the western and 
south-western faces are very precipitous, the cliffs of bare 
rock rising more than 1000 feetatseveral places . . . The 
mass seems to be a huge block of porphbyrite resting on 
shales, erosion having removed all traces of strata from the 
sides, and obliterated all evidence of the manner in which 
the eruptive was once covered by them.” Of the San 
Miguel Mountains of Colorado, Mt. Wilson reaches a height 
of 14,280 feet, and Holmes is quoted as reporting that “all 
that portion of the summit that rises above 11,200 feet 
(3080 feet) is of trachyte. This is underlaid by Cretaceous 
shales in a horizontal position.” 
Many other instances of the actual conditions disclosed 
by systematic observation throughout the wide area 
described are given with full detail in this interesting paper. 
A few brief extracts may be cited from the remarks on the 
character of the igneous rocks. The laccolites of the Henry 
Mountains are described as consisting practically of a single 
rock-type. ‘‘It is a holo-crystalline porphyry, characterised 
by phenocrysts of plagioclase, with hornblende or augite, and 
by a granular ground-mass consisting chiefly of orthoclase 
and quartz. The rock is what has hitherto been called por- 
phyrite by American petrologists, and by some Europeans.” 
The West Elk Mountains show from the character of the 
intrusive rock that “they are practically identical with the 
