56 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. IMe5IOIRS [ vol.xTx; 



holds that the anorthosite is the residual after the separation gravitationally of thefemic minerals 

 from the gabbro-magna in situ. The anorthosite was not ab initio a collection of crystals, but 

 molten, as shown by the evidence of some heterogeneity, and of primary (fluxional) banding, 

 though some granulation occurred in the late stages of consolidation. The syenite was a dis- 

 tinctly later product, does not show gradual passage into the anorthosite, but is clearly intru- 

 sive into the gabbroid margin and the anorthosite, from which it is separated either sharply or 

 by a narrow zone of gneissic hybrid rock. 



Paleozoic. — Very different, however, are the intrusions of peridotite and serpentine in the 

 eastern portions of Canada and the United States. Those of New Brunswick and New England 

 are usually referred to folding in pre-Cambrian times (Van Hise and Leith '09) and so also 

 are some of the intrusions of the eastern townships of Quebec (Dresser '13), but the evidence 

 of age is often obscure. Extending down the highland zone from Newfoundland into North 

 Carolina there is a series of lenticular masses of more or less altered peridotites among schist, 

 some of the peridotites being possibly pre-Cambrian, others Paleozoic, such for example as the 

 serpentines of Broughton in the eastern townships of Quebec in which the intrusion may have 

 occurred between Cambrian and Ordovician times. After the apparently unbroken Ordovician, 

 Silurian, and Lower Devonian sedimentation, the Thetford series of peridotites, pyroxenites, 

 and gabbro were intruded, according to Dresser ('13), during a period of elevation. Harvie 

 ('13) states that they form more or less concordant intrusive sheets, in which peridotite, 

 pyroxenite, gabbro, and diabase pass by gradual or rapid transition into one another. In places 

 the diabase at the outer edge passes into hornblende-granite and aphte which forms indis- 

 tinctly bounded segregation veins, etc. (cf. Dresser '21). These are arranged in order of 

 decreasing basicity and density in sheets from the base upwards, and in batholiths from the 

 center outwards. According to more recent work cited by Bowen ('20), however, the basic 

 rocks are in synclinal sheets, and the granite "batholiths" in the anticlines. A valuable sum- 

 mary of the whole belt of intrusions is given by Lewis and Pratt ('05). (See fig. 17.) 



Beginning in Tallapoosa County, Alabama, where the gneisses and crystalline schists emerge from beneath the 

 Cretaceous and later formations lying farther south, and passing northward one finds that small disconnected peridotite- 

 outcrops occur at short intervals forming a narrow belt that extends approximately N. 50° E. passing near Atlanta, 

 Asheville, Lynchburg, Washington, Trenton, Baltimore, and Philadelphia to New Jersey, where the crystalline rocks 

 again pass under younger formations. With the reappearance of the crystallines at Hoboken, New Jersey, and on 

 Staten Island the peridotites are again represented by large masses of serpentine. A number of other outcrops of ser- 

 pentine, and, in some cases unaltered peridotites, occur through Connecticut and Rhode Island, and Massachusetts 

 and again form a continuous line through the crystalline belt of central Vermont and its continuation through south- 

 eastern Quebec, approximately parallel to the St. Lawrence River, into the Gaspe Peninsula. Large areas of serpentine 

 are known in western Newfoundland and as these are doubtless a part of the same series, the belt of outcrops is extended 

 to a length of over 2,000 miles * * * Systematic petrographers often classify the peridotites as subordinate fades 

 of gabbro, but in North Carolina and in some other portions of the belt this relation is reversed, the gabbros occur only 

 in small masses forming insignificant local facies of the peridotites. * * * In the majority of cases the outcrops are 

 found to have lenticular or elliptical outlines with their longer axes conforming to the direction of lamination of the 

 inclosing gneisses * * * Occasionally a nearly uniform width of outcrop can be traced to relatively great length, 

 indicating sheet-like masses * * * In several instances the lenticular masses are distinctly bifurcate * * * 

 Narrow, fingerlike, or curved and irregular apophyses branch off through the gneisses. The lamination of the gneisses 

 almost invariably follows the outline of the peridotite-mass. The few instances where they have been found to meet 

 the boundaries at a perceptible angle are quite exceptional * * * These relations are what would be expected 

 from the intrusion of a molten magma into a highly laminated rock. That these rocks occupy planes of weakness in the 

 gneisses along which subsequent movement has taken place is strongly suggested by the universal development of 

 schistose secondary minerals about the borders of the peridotites. 



It will be noticed that with the omission of the word "subsequent" a striking agreement 

 with Suess' generalization is here adopted. It is not clear, however, that such movement is 

 necessary to develop schistosity. It may have resulted from movements during consolidation 

 though later movements undoubtedly took place. 



The data are not sufficient exactly to determine the age of these ultrabasic rocks. The 

 zone was one of movement in several epochs. Lewis and Pratt consider " the principal period 

 of intrusion was closely associated with the great orogenic movements of the revolution at the 



