Editorial Comment. 177 
from these to glauconitic sands, the Upper Cretaceous of 
Arkansas, Alabama and Now Jersey. In the light of these 
facts, based upon investigations in the typical regions of the 
occurrence of the North American Cretaceous, the two great 
formations of this country can not be considered of non- 
chalky origin, as was formerly supposed from studies based 
upon the uppermost or latest beds of New Jersey and Alabama, 
but each of them culminates in a great extensive, chalky hori- 
zon, gradating into and included between shallower chalk 
marls," calcareous and glauconitic sands and other more or 
less shallow water sediments. In other words, in the superb 
Cretaceous exposures of the Texas-Arkansas region we have 
recorded in these sediments of the two Cretaceous formations 
the two subsidences heretofore described, and data for much 
progressive research in the future, and which when applied to 
the more disturbed regions to the eastward and westward will 
be of great assistance in interpreting their structure and 
history. 
EDITORIAL COMMENT. 
SOME RECENT WORK UPON THE CRYSTALLINE ROCKS. 
Prof. Judd has recently published three papers of considera- 
ble geological interest to students of the crystalline rocks. In 
the Geological Magazine (June 1889) he discusses the question 
of raetamorphism, insisting on the importance of that phase 
of the subject which he well terms statical metamorphism 
as distinguished from the dynamical metamorphism of Rosen- 
busch. By the former term he means the sum of the changes 
produced in rocks by the included water and gases aided by 
heat and pressure but without movement. The latter term 
comprises the effects of these same agents when accompanied 
with motion. Prof. Judd defines the two classes thus : 
1. Dynamical metamorphiem includes ; — 
Production of cleavape-structure and jointing. 
Crushinjr or deformation of included minerals. 
"Stretching" of the rock.s and production of "mylonitic" bands. 
Changes of various kinds in the constituent minerals of a rock 
without loss of identity. 
' In most of the Rocky Mountain regions and especially in the Trans- 
Pecos and Eagle Pass regions these calcareous marls and ."^ands are 
more consolidated, probably in places the result of igneous metamor- 
phism. At Eagle Springs, Texas, and many other places they can be 
seen capped by a great rhyolitic mass. 
