788 



SCIENCE 



[N. S. Vol. XXVII. No. 693 



a coarse-grained gray-to-pink biotite-gneiss 

 and the other a highly acid pink muscovite- 

 gneiss of finer texture. The latter was found 

 intruded into the former. 



A variant group of semi-metamorphosed 

 basic intrusives is somewhat younger than the 

 gneisses. These include syenites, gabroid 

 and dioritic rocks. In some of these rocks a 

 gneissic structure has been induced, while 

 others are not notably altered. It is evident 

 that they are of different ages relatively to 

 each other, although belonging to the same 

 general interval of time in the section. 



In the northern part of the Sherman quad- 

 rangle gray anorthosite, or labradorite rock 

 (with or without hornblende), is exposed over 

 wide areas. It is intruded into the schists, 

 granite-gneiss and dioritic rocks, but is itself 

 essentially unaltered. 



On the east side of the district the gneisses 

 are further intruded by a quartz-porphyry, 

 which is so little altered that it is believed to 

 be younger than the basic intrusions, although 

 it may be older than the anorthosite. 



All of the foregoing formations are sur- 

 rounded and probably underlain by the great 

 mass of coarse red granite, to which the name 

 " Sherman granite " has been given. The con- 

 tact surface is in many places so nearly hori- 

 zontal that bodies of gneiss and schist form 

 outliers on hill-tops, while the valleys have 

 been trenched through into the younger gran- 

 ite. Dikes of all sizes emanate from this 

 foundation and traverse gneisses, schists, por- 

 phyries and gabbros indiscriminately. The 

 granite itself is essentially imaltered, except 

 that it is deeply weathered at the surface. It 

 is crossed by a moderate number of small 

 dikes, consisting of fine-grained granite, peg- 

 matite and diabase. The diabases are appar- 

 ently the youngest rocks of the pre-Cambrian 

 complex. 



Since there are no well-defined sedimentary 

 rocks in the pre-Cambrian mass, it is not 

 possible to assign the rocks to any particular 

 age. They are covered imconformably by the 

 Carboniferous, but they are so nearly identical 

 with granites and older rocks, which to the 

 north in the Big Horn Mountains and to the 

 south in Colorado lie beneath the Cambrian, 



that the writer has no hesitation in consider- 

 ing them all to be of pre-Cambrian age. It 

 is probable that the schistose complex of vol- 

 canics and intrusives, with traces of sedimen- 

 tary rocks, belongs to the Archean; it is 

 equally probable that the unaltered granites 

 and diabase are not older than the Algonkian. 

 The position of the intermediate formations 

 is entirely problematical. 



Eliot Blackwelder 

 Untvebsitt of Wisconsin 



a statistical study of brown scale 

 parasitism 



Parasitic and predatory enemies are impor- 

 tant factors that should be taken into consid- 

 eration in the control of injurious insects; but 

 the effectiveness of such agencies is very often 

 overestimated, since their efficiency is usually 

 based on estimates made instead of upon accu- 

 rate and sufficient data. It very frequently 

 happens that a notable decrease in numbers 

 of insects occurs coincidently with the in- 

 troduction of a parasite, but this reduction 

 should not be attributed to the parasite alone, 

 as is often the case, since there are many other 

 factors entering into the problem. 



The brown scale (Eulecanium aremniacum 

 Craw) is one of the most important pests of 

 prune trees occurring in this state, and ac- 

 cording to statements frequently made and 

 published here, this scale is kept in very com- 

 plete subjection by its parasite {Comys fusca). 

 This control for the whole state is usually 

 estimated at 95 per cent. During the past 

 winter the writer visited sixty-six different 

 orchards, covering all the important prune 

 sections in the state, and has made an exam- 

 ination of a total of 63,700 scales. From this 

 count the actual percentage of parasitized 

 scales is 12.02 per cent. The counts were 

 made in units of 100 and covered various 

 parts of the tree and various trees in the 

 orchard. The location of each orchard and 

 the name of its owner were taken, and the 

 abundance of scales, both young and adult, 

 in relation to the amount of parasitism, was 

 noted. A summary of a part of these data 

 is given in the table below: 



(I 



