MAUYLAXD GEOLOGICAL SURVEY • 197 



During the most severe winters the Patuxent Eiver is frozen over occa- 

 sionally as far down as its mouth, thus blocking navigation. In Jan- 

 uary, 1893, Dr. Marsh stated : " On the night of the 10th the Patuxent 

 River froze over at its mouth for the first time in 12 years, and it con- 

 tinued frozen until January 30." In February, 1895, the river was 

 frozen over down to its mouth, remaining closed until the 24th. Similar 

 facts were noted in February, 1899, and in January, 1905. 



During the cold wave of February, 1899, the minimum was below 20° 

 at Solomons for 9 consecutive days (9th to ICth), and this is the only 

 period during which temperatures below zero were ever recorded at the 

 station, viz., February 10, 5° below zero; 11th, 3° below zero. The 

 record at Solomons from February 9 to 16, 1899, in comparison with 

 Sunnyside, Garrett County, is shown in Figure 10. 



The Advent of Spring. 



The date of the advent of spring for Calvert County is given as ]\Iarch 

 21, by Mr. F. J. Walz, in his report on the Meteorology and Climatology 

 of Maryland (Vol. I, page 487). This was based on the fact that about 

 this time the daily mean temperature remains permanently above 44°. 

 From the point of view of agricultural interests, the dates of the last 

 killing frost in spring, and the first killing frost in autumn are of 

 much practical importance, and often exert a marked influence on the 

 yield of crops. As a killing frost is defined as one which will cause the 

 death of relatively hardy vegetation, including nearly all the plants of 

 ordinary cultivation, the average date of the last killing frost in spring 

 seems to afford a better criterion of the advent of spring than the arbi- 

 trarily assumed mean temperature of 44°." At Solomons this date is 

 April 8. The average date of the first killing frost in autumn is 

 November 13, so that the duration of the growing season is on the aver- 

 age 219 days. The length of the crop season does not fluctuate very 

 much on account of proximity to the waters of the Chesapeake. The 



' U. S. Weather Bulletin No. 31, p. 69. 



